The German Patent Application 32 01 750, Katona et al, published Sep. 16, 1982, discloses a method for making a light-transmissive sintered body from alumina, to which magnesium oxide in a quantity of from 0.02% to 0.5% by weight has been added as a sintering aid for control of the grain growth. High mechanical strength and high transparency are obtained by additionally adding from 0.005% to 0.1% by weight or zirconium oxide or hafnium oxide or cerium oxide.
The German Patent Application 20 42 379, Kobayashi et al, published Mar. 18, 1971, discloses a light-transmissive alumina sintered body for discharge vessels of high-pressure discharge lamps and a method for making the sintered body. It is doped with from 0.01%-0.1% by weight of MgO, 0.05%-0.5% by weight of Y.sub.2 O.sub.3 and 0.05%-0.5% by weight of La.sub.2 O.sub.3. Satisfactory transparency and high mechanical strength are desired because the sintered body is exposed to alternating thermal loads in operation of the high-pressure discharge lamp.
U.S. Pat. No. 4,285,732, Charles et al, to which published German Patent Application 31 08 677 corresponds, discloses a sintered body and a method for making it. The sintered body comprises, substantially, alumina doped with from 0.03%-0.15% by weight of MgO and with from 0.002%-0.07% by weight of ZrO.sub.2 and/or 0.003%-0.12% by weight of HfO.sub.2. MgO does not only serve to control the grain growth but also provides for a high degree of translucency and eliminates the residual porosity of the ceramic. The other two oxides prevent precipitation of the spinel phase (a mixed crystal of Al.sub.2 O.sub.3 and MgO) and, in addition, regulate the grain growth.
These sintered bodies are particularly advantageously used with high-pressure sodium discharge lamps (see the U.S. Pat. No. 4,567,396, McVey, and U.S. Pat. No. 4,580,075, Strok).
The disclosures of the referenced patents:
are hereby expressly incorporated by reference.
The other publications, some of which are more than a decade old, further describe what is well known in this field.